This invention relates to an electrical signal filter, and particularly is directed to an electrical signal filter for improving electrical characteristics and reliability of the electrical signal filter, for use in a cable television in particular, by protecting it from moisture.
A cable television (hereinafter referred to as CATV) has multiple channels and a viewer enters into a contract with a cable television broadcast service and the television signals are then transmitted to his/her household. Some of these channels can be viewed upon the payment of an additional subscription fee. Normally, channels for which an additional fee is to be paid are provided with electrical signal filters (trap filters) in a cable of the CATV, so that signals having a particular frequency are cut. The system is configured so that the channel for which said additional payment has been made is supplied to the household after this electrical signal filter has been removed.
FIG. 3 explains a conventional electrical signal filter. As shown in the drawing, an electrical signal filter is comprised of a first circuit board 2, a second circuit board 3, a shield member 4, and a shield case 5. Filter circuits are provided to each of the first and second circuit boards 2 and 3, so that signals of channels having a particular frequency are cut thereby.
The shield member 4 is made of a metal plate and accommodated between the first and second circuit boards 2 and 3. The shield member 4 is to function to electro-magnetically isolate the filter circuits provided in each circuit board 2 and 3 in the shield case 5.
The first circuit board 2, the second circuit board 3 and the shield member 4, arranged in line, are provided in the shield case 5.
The shield case 5 consists of an inner case 5a and an outer case 5b. A female connector 6 is provided to the inner case 5a. A male connector 7 is provided to the outer case 5b.
In a configuration of the conventional electrical signal filter, the filter circuits which are composed of coils and condensers are merely soldered on the first and second circuit boards 2 and 3, respectively. As described in above, the conventional electrical signal filter is simply dually shielded by the inner and outer cases 5a and 5b, as well as a Printed Circuit Board. This is insufficient in terms of a protection from moisture. Since the signal filter and its accompanying cables are commonly used in the outdoor and thus susceptible to an influence of moisture, there are always some problems in the electrical characteristic and the reliability of the signal filter.